A method for ascertaining the autarchy time is discussed in German patent document DE 195 17 698 A1. This provides that after switching off the system critical to safety, the end of the autarchy time is recorded, and after expiration of the autarchy time, a still available triggering readiness time is ascertained during which the system could still be kept in triggering readiness by an auxiliary power source, and, in response to switching on the system critical to safety, the autarchy time valid for the current operating time is ascertained from the autarchy time valid during the last operating time and the triggering readiness that was ascertained after the last switching off and is still available. The capacitance of the energy reserve is determined in this instance. But the ascertained current usage or the ascertained supply voltage may also be used in this case.